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Abstract

Background

Although carbohydrate reduction of varying degrees is a popular and controversial
dietary trend, potential long-term effects for health, and cancer in specific, are
largely unknown.

Methods

We studied a previously established low-carbohydrate, high-protein (LCHP) score in
relation to the incidence of cancer and specific cancer types in a population-based
cohort in northern Sweden. Participants were 62,582 men and women with up to 17.8 years
of follow-up (median 9.7), including 3,059 prospective cancer cases. Cox regression
analyses were performed for a LCHP score based on the sum of energy-adjusted deciles
of carbohydrate (descending) and protein (ascending) intake labeled 1 to 10, with
higher scores representing a diet lower in carbohydrates and higher in protein. Important
potential confounders were accounted for, and the role of metabolic risk profile,
macronutrient quality including saturated fat intake, and adequacy of energy intake
reporting was explored.

Results

For the lowest to highest LCHP scores, 2 to 20, carbohydrate intakes ranged from median
60.9 to 38.9% of total energy intake. Both protein (primarily animal sources) and
particularly fat (both saturated and unsaturated) intakes increased with increasing
LCHP scores. LCHP score was not related to cancer risk, except for a non-dose-dependent,
positive association for respiratory tract cancer that was statistically significant
in men. The multivariate hazard ratio for medium (9–13) versus low (2–8) LCHP scores
was 1.84 (95% confidence interval: 1.05-3.23; p-trend = 0.38). Other analyses were
largely consistent with the main results, although LCHP score was associated with
colorectal cancer risk inversely in women with high saturated fat intakes, and positively
in men with higher LCHP scores based on vegetable protein.

Conclusion

These largely null results provide important information concerning the long-term
safety of moderate carbohydrate reduction and consequent increases in protein and,
in this cohort, especially fat intakes. In order to determine the effects of stricter
carbohydrate restriction, further studies encompassing a wider range of macronutrient
intakes are warranted.

Keywords:

Introduction

In recent years, low-carbohydrate diets have emerged as a controversial and popular
means of achieving weight loss and controlling diabetes. In Sweden, extensive positive
media support for dietary carbohydrate restriction has occurred over the past 5–7 years
[1]. During the same time period, in northern Sweden, a complete reversal of previous
reductions in fat intake and cholesterol levels has been reported in the general population
[2,3]. Discerning the potential long-term health effects of carbohydrate restriction, not
only of stringent low-carbohydrate diets but also of more modest carbohydrate reduction,
is thus an important challenge in nutrition research today.

For weight loss, low-carbohydrate diets, both extremely or more modestly reduced in
carbohydrate (e.g. E% carbohydrates/protein/fat = 9/28/63 [4], and 44/18/38 [5], respectively) have been found to be at least as effective as traditional low-calorie/low-fat
diets over a period of up to two years [5-7]. The results of randomized trials have also tended to support improved metabolic
parameters and blood lipids [8-11], but elevated markers of stress and inflammation [11-13] in subjects following a low-carbohydrate diet. These alterations might influence
the risk of major chronic diseases such as cardiovascular disease and cancer [11,14]. However, from a long-term perspective, the effects of carbohydrate reduction of
varying degrees, and consequent increased consumption of various types of protein
and/or fat, for health outcomes, and cancer in specific, are largely unknown.

The results of previous epidemiological studies in general populations have suggested
positive or null associations between low-carbohydrate diet scores, particularly scores
representing diets higher in foods of animal origin, and all-cause, cardiovascular,
and cancer mortality [15-19]. Prospective studies of cardiovascular disease incidence have reported either an
increased risk [20], or reduced risk for plant-based [21], carbohydrate-restricted diets. The only previous prospective study to address overall
cancer incidence found null associations for both animal- and plant-based low-carbohydrate
diets [22]. An increased risk of incident breast cancer has been observed for a dietary pattern
characterized by a low intake of bread and fruit juice and a high intake of processed
meat, fish, butter, other animal fats, and margarine [23]. In contrast, a plant-based, low-carbohydrate diet has been related to a reduced
risk of estrogen-receptor-negative breast cancer [24].

Given the high rates of overweight and obesity worldwide, and the widespread popularity
of low-carbohydrate diets, evaluation of the long-term safety of carbohydrate restriction
of varying degrees is crucial. The aim of the present study was to investigate macronutrient
distribution, in particular a previously established low-carbohydrate, high-protein
(LCHP) score [16-20], in relation to the risk of incident cancer and specific types of cancer in a large,
population-based cohort in northern Sweden.

Methods

Study design and cohort

The Västerbotten Intervention Programme (VIP) is an ongoing, population-based, prospective
cohort study and health intervention, including residents of the northern Swedish
county of Västerbotten turning 40, 50 and 60 years of age. Until 1996, 30 year olds
were also included. The VIP protocol, described in detail elsewhere [25,26], includes a health examination, with measurement of a number of potential health
risk factors, such as an oral glucose tolerance test, as well as a participant-administered
diet and lifestyle questionnaire. For the period assessed in this study, 1990–2007,
the average recruitment rate was 59% . The VIP food frequency questionnaire (FFQ)
has been validated by 24-hour-recall interviews [27], and by biomarkers in blood samples collected from VIP participants [28,29]. Cancer incidences comparable to those of the general population in Västerbotten
indicate a truly population-based cohort [30], and no selection bias of importance has been demonstrated [31].

As of December 31, 2007, when cases of incident cancer were identified for the present
study, a total of 82,879 participation occasions (66,001 individuals) had been registered
within the VIP cohort. From these we excluded 1,328 participation occasions with missing
data for more than 10% of the items in the FFQ and/or portion size, 32 participation
occasions with missing height or weight data, 9 participation occasions with a body
mass index (BMI) <10, and 6,112 participation occasions with food intake level (FIL)
in the lowest 5th percentile or the highest 2.5th percentile (specific to sex and
FFQ version and based on the first sampling occasion for subjects with repeated measures),
and 12,816 participants with more than one sampling occasion (most recent sampling
occasion excluded). The final study population thus included 62,582 participants (31,397
men, 31,185 women).

Low-carbohydrate, high-protein score

Dietary intake of macronutrients was calculated from food frequency questionnaires
with 9 fixed response alternatives ranging from never to ≥4 times per day and including
84 (years 1990–1996) or 65 (years 1997–2007) food items, as well as photo-based portion-size
estimations for meat/fish, potatoes/pasta/rice, and vegetables [26]. The 65-item FFQ was an abbreviated version of the 84-item FFQ, in which some food
items had been merged and some deleted as described elsewhere [33] (p 26). All intake variables except ethanol were energy adjusted according to the
residual method [34].

Descending deciles, or tenths, of energy-adjusted carbohydrate and ascending deciles
of energy-adjusted protein were labeled 1 to 10 and summed to create an LCHP score
(2–20 points), a model employed in several previous studies [16-20]. The LCHP score is independent of total energy intake, due to the isocaloric nature
of carbohydrate and protein, and it allows separate consideration of the amount and
quality of fat consumed. LCHP scores were also categorized into low (2–8 points),
medium (9–13 points) and high (14–20 points), in order to approximate equally sized
groups.

Statistical analyses

Differences in baseline characteristics of the study subjects according to LCHP score
category were determined by the Kruskal-Wallis test. Spearman’s correlation coefficients
were determined between LCHP score and intakes of fat and saturated fat and sex-specific
analyses were done. Hazard ratios (HR) and 95% confidence intervals (CI) for overall
cancer incidence and for all types of cancer with at least 50 cases were calculated
by Cox proportional hazard regression models. HR are presented for medium and high
versus low LCHP scores or per 1-point increase in LCHP score. p-trend were calculated
per 1-point increase in LCHP score. Age and BMI deviated from the proportional hazard
assumption according to Schoenfeld’s test. Age was thus examined in 10-year intervals,
included as strata in the crude and multivariate models, and BMI was dichotomized
according to obesity (BMI ≥30 kg/m2).

Subgroup analyses were conducted for metabolic risk profile, defined as at least one
of, versus none of, obesity, diabetes or impaired glucose tolerance. Diabetes was
defined as fasting plasma glucose ≥7.0 mmol/l and/or post-load plasma glucose ≥12.2 mmol/l,
and impaired glucose tolerance was defined as fasting plasma glucose ≥6.1 mmol/l and/or
post-load plasma glucose ≥8.9 mmol/l. Subgroups based on saturated fat intake (energy
adjusted and stratified at the median) and energy reporting (adequate versus inadequate,
according to the Goldberg cut-off, modified as described in our previous report [19]) were also examined. The subgroup analyses were limited to overall cancer incidence
and cancer of the prostate, breast and colorectum, which were the most common sites.
Heterogeneity was tested by Chi-square analysis. A sub-analysis was also performed
for the time period prior to the shift in macronutrient intake in the VIP population
[2] (follow-up to December 31, 2002). All tests were two-sided, and p-values <0.05 were
considered statistically significant.

Ethical considerations

The study was approved by the Regional Ethical Review Board of Northern Sweden (dossier
number 07-165 M). All study subjects provided written informed consent, and the study
was conducted in accordance with the Declaration of Helsinki.

Results

Follow-up times ranged from 1 day to 17.9 years, median 9.7 years. Macronutrient intakes
for the lowest to highest LCHP scores (2–20 points) ranged from median 60.9 to 38.9%
of total energy intake for carbohydrate, 11.3 to 18.9% for protein, and 26.7 to 41.5%
for fat. Relationships between baseline characteristics and LCHP score are presented
in Table 1. High LCHP scores were associated with younger age (not apparent in medians due to
sampling at 10-year age intervals) and higher BMI, prevalence of current smokers,
sedentary lifestyle (women only) and alcohol intake. Lack of postsecondary education
was more common in men with low LCHP scores and in women with high scores. LCHP scores
were positively related to intake of animal protein, but negatively related to plant
protein. For carbohydrate and fat, associations were consistent in sucrose and whole
grain and saturated and unsaturated fat, respectively. Spearman correlation coefficients
for LCHP score and energy-adjusted fat, saturated fat and unsaturated fat intakes
were 0.51, 0.45, and 0.46, respectively.

There were no statistically significant associations between LCHP score and any cancer,
with the exception of an increased risk of respiratory tract cancer for medium LCHP
scores in men (multivariate HR for medium versus low LCHP scores 1.84; 95% CI 1.05-3.23;
p-trend = 0.38) (Table 2). HR for high versus low LCHP scores for respiratory tract cancer were above one
in both men and women, but not statistically significant.

Subgroup analyses based on metabolic risk profile, saturated fat intake, and energy
reporting [19], had no material effects on results (Table 3). The only statistically significant finding was an inverse association between LCHP
score and colorectal cancer risk in women with high saturated fat intakes (multivariate
HR for a 1-point increase in LCHP score 0.92; 95% CI 0.87-0.98; p = 0.013; p-heterogeneity = 0.003).
Constructing LCHP scores in which whole grain or sucrose replaced total carbohydrates,
and in which vegetable or animal protein replaced total protein intake (data not shown),
also did not differ from the main findings, except a statistically significant increased
risk of colorectal cancer in men with higher LCHP scores based on vegetable protein
(multivariate HR for a 1-point increase in LCHP score 1.07; 95% CI 1.01-1.14; p = 0.029;
p-heterogeneity = 0.016).

Table 3.Associations between low-carbohydrate, high-protein (LCHP) score and incident all-cause
and site-specific cancer in subgroups of participants in the Västerbotten Intervention
Programme based on metabolic risk profile, saturated fat intake, and energy reporting

In analyses restricted to the time period up to and including December 31, 2002, there
was a tendency towards a positive association between high LCHP scores and overall
cancer risk in both men (multivariate HR for high versus low LCHP scores 1.25; 95%
CI 0.86-1.80; p-trend = 0.093) and women, (multivariate HR for high versus low LCHP
scores 1.39; 95% CI 0.98-1.96; p-trend = 0.020) (Table 4). For prostate, breast and colorectal cancers no significant associations were found.

Table 4.Associations between low-carbohydrate, high-protein (LCHP) score and incident all-cause
and site-specific cancer in Västerbotten Intervention Programme participants in a
subgroup with reduced follow-up until 2002

Discussion

In this large population-based cohort study with a follow-up period of up to 17.9 years,
a diet moderately low in carbohydrates and moderately high in protein was largely
unrelated to overall and site-specific cancer incidence, regardless of the quantity
and quality of fat intake.

The one previous prospective study to report results for overall cancer incidence,
from the Iowa Women’s Health Study, reported inverse risk relationships for isocaloric
substitution of either animal or vegetable protein for carbohydrates [22]. However, the results were attenuated to null in multivariate analyses. Associations
reported for overall cancer mortality have also been null, non-statistically significant,
or unstable [15,17,19,22]. Taken together, the evidence to date does not support a role for moderate carbohydrate
reduction in determining the overall risk of cancer.

Increasing LCHP scores were associated with an elevated risk of respiratory tract
cancer in both men and women in the present study, but the relationship was not dose
dependent and was only statistically significant for medium LCHP scores in men. Although
these observations may be due to chance or reflect residual confounding due to smoking,
they are consistent with a previous finding for lung cancer mortality [15]. Further study is therefore warranted.

There are several mechanisms by which a carbohydrate-reduced diet could influence
carcinogenesis, through specific food items or components, such as red and processed
meat for example [35], or through effects on energy metabolism and body composition [36-39]. In analyses considering macronutrient quality and metabolic profile at baseline,
two statistically significant results were observed, an inverse association between
LCHP score and colorectal cancer risk in women with high saturated fat intakes, and
an increased risk of colorectal cancer in men with higher LCHP scores based on vegetable
protein. These findings do not support the hypothesis that high animal protein intake
increases the risk for these cancer types. Previously, we have reported a null association
between LCHP score and colorectal cancer mortality [19], and a positive association has been noted for an animal-based, low-carbohydrate
score and colorectal cancer mortality [15]. The latter finding is more consistent with the current understanding of the role
of diet in colorectal cancer. For example, there is convincing evidence that a high
consumption of protein sources such as red and processed meat is associated with increased
colorectal cancer risk [35]. Furthermore, in a controlled trial, a LCHP weight-loss diet has been observed to
reduce fecal cancer-protective metabolites and increase hazardous metabolites, which
could increase the risk of colon cancer [40].

The limited variability in macronutrient distribution in the study population may
have prevented the detection of associations with cancer risk. In particular, the
role of stricter carbohydrate restriction could not be assessed. This is an issue
common to both the present and previous studies [15,17,19,22]. Interindividual differences, such as gene-nutrient interactions and epigenetics,
both emerging research fields [36], may also complicate the relationship between macronutrient distribution and cancer
risk. Furthermore, carbohydrate restriction might have different roles in different
stages of tumorigenesis, making it difficult to detect overall effects on incidence.
For example, putative mechanisms for a role for carbohydrate in the progression from
premalignant lesion to cancer diagnosis include metabolic reprogramming of cancer
cells resulting in increased glycolysis and glucose requirements, the so-called Warburg
effect, as well as the stimulatory effect of insulin-like growth factor on proliferating
cells [37,41].

In northern Sweden, a rapid decline in fat intake and hypercholesterolemia occurred
between the years 1986–1992 [42], attributed in part to the cardiovascular disease prevention activities of the VIP
[42]. Today fat intake has reached the peak levels of the 1980’s, and carbohydrate intake
is decreasing [2]. LCHP scores have increased in VIP participants with repeated samples 10 years apart
[19]. Furthermore, blood cholesterol concentrations in the northern Swedish population
are increasing, despite increasing use of cholesterol-lowering drugs [3]. These temporal changes may have attenuated our results, as indicated by the positive
association between a high LCHP score and overall cancer incidence in the sub-analysis
restricted to the time period 1990–2002, when LCHP score was relatively stable in
the VIP population. In the present study, roughly equal amounts of saturated and unsaturated
fat replaced most of the carbohydrate reduction in subjects with high LCHP scores,
and the excess protein consumed by subjects with high LCHP scores was primarily of
animal origin. In Sweden, extensive positive media focus for carbohydrate-restricted
diets in recent years has largely promoted fat, often animal fat, rather than protein,
as the substitute for carbohydrates [1,43]. The general enthusiasm for carbohydrate reduction, and the apparent preference for
animal-based replacement foods in Sweden, thus underscores the importance of evaluating
potential long-term implications for health.

The main strengths of this study were the large, population-based cohort, the extensive
data available, such as an oral glucose tolerance test and BMI measured by health
professionals, and the long, essentially complete, follow-up. In addition, the prospective
study design reduced the risk of recall bias and reverse causation. We used an established
LCHP score, which has been employed in previous studies [16-18]. The LCHP score does not include fat intake. However, unlike macronutrient scores
that incorporate fat, the LCHP score is independent of total energy intake. It is
also simple, both to calculate and interpret, and it allowed separate consideration
of the amount and quality of fat consumed. Food frequency questionnaires like the
one employed is this study have inherent weaknesses, such as being a relatively inexact
tool for the measurement of total nutrient and energy intake, but they are generally
adequate for ranking and are an accepted and practical tool for large-scale epidemiological
studies. Although several potential confounders were accounted for, residual confounding
due to factors not measured (such as food items not included in the FFQ) or not adequately
estimated (such as history of tobacco use) was likely present. Since we consider this
study to be exploratory, the results were not corrected for multiple testing. The
risk of chance findings should therefore be acknowledged. Numbers of cases were also
low for some cancer types and in some subgroup analyses.

Conclusion

In conclusion, the results of this population-based, cohort study do not support an
important role for a diet moderately low in carbohydrates and moderately high in protein,
regardless of the quantity and quality of fat consumed, in determining the overall,
long-term risk of cancer, although a possible increased risk of respiratory cancer
was observed and a tendency of an increased general cancer risk over shorter time.
Given the current widespread popularity of carbohydrate-restricted diets, and the
limited data concerning potential long-term health effects of carbohydrate reduction
and consequent increases in protein and/or fat intakes, these findings are important.
In order to evaluate the role of more stringent carbohydrate restriction, investigations
encompassing a wider range of macronutrient intakes, such as multicenter studies,
will be needed.

Competing interests

The authors declare that they have no competing interests.

Authors’ contribution

LMN, AW, IJ, GH, and BVG designed and conducted the research. LMN analyzed the data.
LMN and BVG interpreted the data and drafted the manuscript, and LMN had primary responsibility
for the final content. AW, IJ, BL, GH, and PL contributed important scientific content,
and all authors revised manuscript drafts and read and approved the final version.

Acknowledgments

This work was supported by grants from the Cancer Research Foundation in Northern
Sweden, Nordic Health and Whole grain Food (HELGA)/Nordforsk, the Swedish Research
Council and the Västerbotten County Council. We also acknowledge the Västerbotten
Intervention Programme participants and the VIP diet database team.

Funding

This work was supported by grants from the Cancer Research Foundation in Northern
Sweden, Nordic Health and Whole grain Food (HELGA)/Nordforsk, the Swedish Research
Council and the Västerbotten County Council.